A honeycomb structure made of a ceramics has been used in a catalyst carrier for an internal combustion engine, a boiler, a chemical reaction device, a reformer for a fuel cell and the like in which a catalyst function is used, a trapping filter of fine particles in an exhaust gas, particularly diesel fine particles (a diesel particulate filter, hereinafter sometimes referred to as the “DPF”), and the like.
In general, as shown in FIGS. 2(a) and 2(b), a honeycomb structure for use in such purposes has a structure which includes a plurality of cells 23 constituting fluid channels divided by porous partition walls 24 and in which adjacent cells 23 are alternatively plugged on the opposite end portions so that end faces have a checkered pattern. In a honeycomb structure 21 having such a structure, a fluid to be treated flows into the cell 23 not plugged on an inflow hole side end face 25, that is, the cell 23 with a plugged end portion on an outflow hole side end face 26, and the fluid passes through the porous partition wall 24, and is discharged from the adjacent cell 23, that is, the cell 23 with a plugged end portion on the inflow hole side end face 25 and not plugged on the outflow hole side end face 26. In this case, the partition walls 24 function as a filter. When the structure is used as, for example, the DPF, a particulate matter (hereinafter sometimes referred to as the “PM”) such as soot discharged from a diesel engine is trapped by the partition walls 24 and accumulates on the partition walls 24.
The honeycomb structure used in this manner has a problem such that a temperature distribution in the honeycomb structure becomes nonuniform owing to the rapid temperature change of the exhaust gas or local heat generation, and thereby cracks are generated in the honeycomb structure. In particular, when the structure is used as the DPF, a particulate matter such as the accumulated soot needs to be burnt and removed to regenerate the structure. In this case, local temperature rise is caused, which results in problems that a regeneration efficiency lowers owing to the nonuniformity of a regeneration temperature and that the cracks due to a large thermal stress are easily generated.
In addition, in a case where the ceramic honeycomb structure provided with a plurality of cells extending therethrough is used as a dust-collecting filter for use in a corrosive gas atmosphere at a high temperature, for example, the diesel particulate filter (DPF) which traps the particulate matter (PM) discharged from the diesel engine, owing to the local heat generation accompanying the abnormal burning of the PM, thermal shock brought by the rapid temperature change of the exhaust gas, and the like, the nonuniform temperature distribution is generated in the structure, and the thermal stress acts. As a result, the ceramic honeycomb structure might incur the crack generation or melting.
To solve this problem, a method is suggested in which a plurality of divided segments of the honeycomb structure are bonded with a bonding material. Specifically, a method for manufacturing the honeycomb structure in which a large number of honeycomb bodies (segments) are bonded together with discontinuous bonding materials is disclosed (for example, see Patent Document 1).
Moreover, a method for manufacturing a thermal-shock-resistant rotary heat storage type ceramic heat exchanger is suggested in which the matrix segments of the honeycomb structure constituted of a ceramic material are formed by extrusion, fired, and then the outer peripheral portions of the segments are processed to be flat and smooth. Afterward, the bonding portions of the segments are coated with a ceramic bonding material in which a fired mineral composition is substantially the same as that of the matrix segments and in which a thermal expansion ratio difference is 0.1% or less at 800° C., and they are fired (for example, see Patent Document 2).
Furthermore, a ceramic honeycomb structure is disclosed in which cordierite honeycomb segments are bonded with the same material of cordierite cement (for example, see Non-Patent Document 1).
In the honeycomb structure in which such honeycomb segments are integrated using the bonding material, it is an important theme to secure a bonding strength between the honeycomb segments, but a bonding defect is sometimes generated. For example, bonding layers are cracked owing to the difference of the thermal expansion ratio or a contraction ratio due to the firing between the bonding layers and the honeycomb segments, or the bonding layers themselves peel. In particular, in the honeycomb structure of a large size, especially with a channel (cell) length of 50 mm or more, the difference of the thermal expansion or the contraction due to the firing between the bonding layers and the honeycomb segments remarkably increases. Therefore, there is a problem that it is difficult to obtain a honeycomb structure of a large size in which any bonding defect is not generated in the bonding layers (the bonding portions).
To solve such problems, a honeycomb structure is suggested in which a plurality of honeycomb segments are securely bonded to one another without generating any bonding defect such as crack or peel in the bonding portions of these segments. Moreover, a method for manufacturing a honeycomb structure having such characteristics is suggested. Furthermore, a honeycomb structure made of a ceramic material is suggested in which there a bonding material capable of bonding together bodies to be bonded without generating any bonding defect such as the crack or peel in the bonded portions is used. In the honeycomb structure, a plurality of honeycomb segments provided with cell structures having a plurality of cells constituting fluid channels divided by porous partition walls and porous outer walls on outer peripheries of the cell structures, the bonding material containing colloidal silica or the like is dried to form the bonding layers on the outer walls, and the outer walls are bonded to one another via the bonding layers (see Patent Document 3).
In addition, a ceramic structure is suggested in which a plurality of ceramic members are united to form an aggregate. Each of the ceramic members has a plurality of through holes arranged in a longitudinal direction, and these through holes are plugged on the end faces so that each of the end faces has a checkered pattern. Moreover, the through holes have a reversed opening/closing relation on gas inlet and outlet sides. Furthermore, air can pass through the adjacent through holes via the porous partition walls. In the ceramic structure, a portion between the ceramic members is filled with a material constituted of at least an inorganic fiber, an inorganic binder, an organic binder, and inorganic particles, dried, and hardened to form an elastic seal material having a structure in which the inorganic fiber, the inorganic particles, and a ceramic material formed by heating and firing the inorganic binder three-dimensionally cross one another. The respective ceramic members are integrally bonded via the sealing material. In particular, as the inorganic particles, at least one or more types of inorganic powder or whisker selected from the group consisting of silicon carbide, silicon nitride and boron nitride are used. As the inorganic fiber, at least one or more ceramic fibers selected from the group consisting of silica-alumina, mullite, alumina, and silica are suggested (see Patent Document 4).
With regard to the conventional ceramic fiber, it has been necessary to consider that there is a possibility of affecting a human body when particle diameters, a composition, and an existence form satisfy conditions within certain values. Therefore, to manufacture the honeycomb structure in consideration of health, a new approach different from the above conventional technology has been demanded.
Thus, as a honeycomb structure using a bio-soluble fiber, a honeycomb structure formed of nonwoven cloth using the bio-soluble fiber is suggested. This honeycomb structure is formed by alternately laminating flat-plate-like nonwoven cloth and waveform-like nonwoven cloth, and the nonwoven cloth is formed of the bio-soluble fiber and a binder (see Patent Document 5). Therefore, the presence of the binder is essential for this honeycomb structure of Patent Document 5, and the structure could not be used in the heretofore used honeycomb structure and the bonding material for the structure. Moreover, the structure is thermally resistant to 800° C. or more, and hence the structure cannot sufficiently bear the use at 1200° C. or more as a honeycomb structure made of a ceramics for use as a catalyst carrier for an internal combustion engine, a boiler, a chemical reaction device, a reformer for a fuel cell, or the like in which a catalyst function is used, or as a DPF or the like which traps a PM in an exhaust gas.    Patent Document 1: U.S. Pat. No. 4,335,783    Patent Document 2: JP-B-61-51240    Patent Document 3: JP-A-2005-154202    Patent Document 4: JP Patent No. 3121497    Patent Document 5: JP-A-2003-105662    Non-Patent Document 1: SAE Paper 860008 (1986)